A day after President Obama became the first holder of the Oval Office to support same-sex marriage, Mitt Romney apologized Thursday for “some dumb things’’ he did in high school after he was accused of bullying a gay schoolmate as a teenager in 1965.

According to a Washington Post report, Romney once led a posse that attacked a boy named John Lauber, who was “perpetually teased for his nonconformity and presumed homosexuality,’’ pinning him to the floor while Romney cut the boy’s bleached-blond hair, which usually draped over one eye.

The Post cited five men who attended the Cranbrook School in Bloomfield Hills, Mich., with the presumptive Republican nominee.

One of the men, Phillip Maxwell, told ABC News that there was no doubt the episode was bullying.

“Oh my god, are you kidding?’’ said Maxwell, now a lawyer who is still friends with Romney. “I castigated myself regularly for not having intervened.

“When I saw the look on his [Lauber’s] face, it was a look I’ll never forget,’’ he said. “This was bullying supreme.’’

Schoolmates also said Romney frequently shouted “Atta girl!’’ when another gay student, Gary Hummel, spoke in class, according to the Post.

In an interview with conservative radio host Brian Kilmeade, Romney said he did not recall such incidents but added, “I participated in a lot of hijinks and pranks in high school and some may have gone too far, and for that I apologize.’’

Romney - who as governor of Massachusetts eliminated the Governor’s Commission on Gay and Lesbian Youth, a program designed to end harassment of homosexual teens - insisted that whatever he might have done was not motivated by antigay sentiments. He noted that both Lauber and Hummel were in the closet at the time and said he had “no idea’’ they were gay.

The Post said Lauber died in 2004.

In an interview with the Globe, Sidney Barthwell Jr., a Romney classmate who did not participate in the Post report, said friends told him about Lauber’s violent haircut.

The only African-American in the Cranbrook class of 1965, Barthwell said that Romney and his family were known at the time as civil rights proponents. He said that there were several “effeminate’’ students at the all-boys school and that they were sometimes addressed “using terms that would not be politically correct today.’’ But he never heard such language from Romney.

“No, not at all,’’ Barthwell said.

Barthwell said the mistreatment of Lauber was more likely based on his demeanor and hair-do than his sexual orientation.

Schoolmates also told the Post that on more than one occasion Romney humiliated an English teacher by exploiting his near-blindness.

Romney and his backers have responded to the anecdotes with a two-pronged defense: Romney has changed, and decades-old antics have no place in contemporary political discourse.

“I am quite a different guy now that I am married, have five sons, five daughters-in-law, and 18 grandchildren,’’ Romney said. “I think at some point in this campaign we’ll talk about things like the economy and energy, labor policy, Syria, Iran. I am hoping at some point that mainstream media is able to get around to those topics.’’

Craig Baker, a registered Republican from Kingston, called the bullying report “character assassination’’ and labeled it an example of liberal bias in the press.

“I think the timing is quite underhanded, number one,’’ said Baker, a 43-year-old engineer who “has no problem with gay marriage.

“Number two, I think that people looking at something that happened in 1965 is not relevant at all.’’

Callum Borchers can be reached at callum.borchers@globe.com. Follow him on Twitter @callumborchers.